Physiology of pregnancy

Question 1

What is the duration of an average human pregnancy and how long is each trimester?

Answer 1

40 weeks / 9 months

Three trimesters - first (<12 weeks), second (12 weeks to end week 26), third 27 weeks - birth

Question 2

What are the physiological changes that occur during pregnancy to support the needs of a growing foetus?

Answer 2

Supplying a baby with enough nutrients (O2, glucose, amino acids) for growth, development, production of amniotic fluid.
Managing increased waste production – eg. CO2, nitrogen compounds.
Change in hormones to support pregnancy and prepare for delivery.
Anatomical changes to accommodate the growing fetus and preparing for labour.
Manage the stresses of delivery and potential haemorrhage.
Postnatal recovery and breastfeeding.

Question 3

What are the 3 main hormones that are involved in pregnancy?

Answer 3

BHCG – dramatic rise in the first days-weeks, released from corpus luteum initially, then by placenta.
Progesterone eg. pro-gestation - keeps the state of quiescence.
Oestrogen – breast growth, areolar enlargement.

Question 4

What produces hCG (human chorionic gonadotrophin), when is it first detectable and when does it peak?

Answer 4

produced by trophoblast, first detectable 8-9 days, peaks 8-9 weeks

Question 5

What is the function of hPL (human placental lactogen)?

Answer 5

Alters maternal carbohydrate and lipid metabolism to provide steady state of glucose for fetal requirements

Question 6

What is the function of hPG (human placental gonadotrophin)?

Answer 6

induces maternal insulin resistance to regulate fetal growth which can become a pathological process seen as GDM

Question 7

What are the protein hormones involved in pregnancy?

Answer 7

hCG, hPL, hPG

Question 8

What are the steroid hormones involved in pregnancy?

Answer 8

oestrogen (oestriol), progesterone.

Question 9

What is the function of hCG?
When does hCG decrease?
What are the functions of the beta and Alpha units?
When are large quantities of hCG released?
What are the consequences of high levels of hCG?

Answer 9

maintains corpus luteum secretion of prog and oest. decreases as the placental production of progesterone increases.
beta unit forms the basis of pregnancy testing, Alpha unit can mimic LH, FSH, and TSH.
large quantities are released in molar pregnancy and multiple pregnancy.
high levels cause vomiting (hyperemesis).

Question 10

What are the functions of progesterone during pregnancy?

Answer 10

Relaxes smooth muscle everywhere.
Maintains uterine quiescence by decreasing uterine electrical activity.
Constipation, gastric reflux, supra-pubic dysfunction.
Immune suppressor (HLA).
Lobulo-alveolar development in breasts.
Substrate for fetal adrenal corticoid synthesis e.g cortisol.

Question 11

What are the functions of oestrogen during pregnancy?

Answer 11

Growth of the uterus, cervical changes
Development of ductal system of breasts
Stimulation of prolactin synthesis

Question 12

How does pregnancy affect the haematological system?

Answer 12

40% increase in plasma volume, 25% increase in RBC -
leads to dilutional anaemia.
Plasma colloid osmotic pressure falls – shift of fluid into extracellular space
Increase clotting factors – hypercoaguable state, evolutionary balance between thrombosis and haemorrhage, increase plasma fibrinogen (increased ESR), platelets, factor VIII & von willebrand factor.

Question 13

How does pregnancy affect the circulatory system?

Answer 13

Increased blood volume has implications on:
cardiac output (increased), peripheral resistance (decreased), blood pressure.
Heart is working hard reach supply demands of the fetus – increase in heart rate and stroke volume accounts for increase in cardiac output.
Heart enlarges (increased venous return).
Innocent systolic murmurs are common.
Beware diastolic murmur – may signify cardiomyopathy.

Question 14

How does pregnancy affect peripheral vascular resistance?

Answer 14

Peripheral vasodilatation (effect of progesterone).
Peripheral resistance decreases, combined with increased cardiac output, results in slightly lower BP.
Decreased vascular resistance leads to lower blood pressure.

Question 15

How does pregnancy affect the respiratory system?

Answer 15

Increased oxygen demands.
Decrease maternal pCO2 and increase maternal pO2.
Increased availability of O2 to tissues and aids passive diffusion at placenta, HbF has higher oxygen affinity.

Question 16

What are the symptoms/signs of pregnancy?

Answer 16

Splinting of diaphragm, increased ventilation – sensation of increasing SOB.
Raised HR leads to palpitations.
Lots of cross over with symptoms for PE, hypercoagulable state.
Excess plasma volume shifts causing peripheral oedema.
Decreased exercise tolerance.
Low BP causing fainting / dizziness.

Question 17

What are the effects on pregnancy on the musculoskeletal and dermatological systems?

Answer 17

Increased lumbar lordosis
Ligamentous laxity – pelvic girdle pain / pubis dysfunction
Stretch marks
Changes in skin pigmentation - Linea Nigra, melasma, darkened nipples
Carpal Tunnel
Sciatica
Cramps

Question 18

How does pregnancy affect the breasts?

Answer 18

oestrogen increases adipose tissue causing enlargement and areolar enlargements and the ductal system. progesterone enlarges breast lobules.

Question 19

How does pregnancy affect urology?

Answer 19

Kidney increases 1cm in size during normal pregnancy,
increased renal flow by 50%, increased GFR (BUT tubular reabsorption capacity is unchanged) - decreased glucose reabsorption - glycosuria is common.
Plasma levels of creatinine and urea decrease.
Dilated ureters (progesterone).
Increased pressure (increased urine frequency).

Question 20

How does pregnancy affect the thyroid gland?

Answer 20

Increased serum T3 and T4 levels, increase in thyroid binding globulin (oestrogen).
As only unbound T3 and T4 is active, levels of free T3 and T4 remain the same or fall slightly.